Air filter with laminated polytetrafluoroethylene membranes

ABSTRACT

An air filter includes at least a laminate having porous first and second polytetrafluoroethylene membranes and an air-permeable supporting member that is sandwiched between and that is bonded to the first and second polytetrafluoroethylene membranes.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an air filter having laminated porouspolytetrafluoroethylene membranes.

[0003] 2. Description of the Related Art

[0004]FIG. 1 illustrates a conventional high performance air filterwhich includes a polytetrafluoroethylene (PTFE) membrane 12 laminatedwith a supporting member 11. The air filter is particularly useful forcollecting suspended particles in a clean room. Performance of the airfilter can be expressed by collecting efficiency and pressure drop. Thecollecting efficiency of the air filter is defined as percentage of thenumber of removed particles divided by the total number of particlesprior to filtration. While exhibiting high collecting efficiency(greater than 99.99%), the pressure drop of the conventional air filteris relatively high, which, in turn, results in a decrease in air flowrate during operation.

SUMMARY OF THE INVENTION

[0005] Therefore, it is an object of the present invention to provide anair filter for collecting suspended fine particles in a clean room thatis capable of overcoming the aforementioned drawback.

[0006] According to the present invention, there is provided an airfilter for collecting suspended fine particles in a clean room. The airfilter comprises at least a laminate including porous first and secondpolytetrafluoroethylene membranes and an air-permeable supporting memberthat is sandwiched between and that is bonded to the first and secondpolytetrafluoroethylene membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In drawings which illustrate embodiments of the invention,

[0008]FIG. 1 is a cross-sectional side view of a conventional airfilter;

[0009]FIG. 2 is a cross-sectional side view illustrating a firstpreferred embodiment of an air filter of this invention;

[0010]FIG. 3 is a cross-sectional side view illustrating a secondpreferred embodiment modified from the air filter of FIG. 2; and

[0011]FIG. 4 is a cross-sectional side view illustrating a thirdpreferred embodiment modified from the air filter of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]FIG. 2 illustrates a first preferred embodiment of an air filterof this invention for collecting suspended particles in a clean room.The air filter includes a laminate that has porous first and secondfiltering membranes 3 and an air-permeable supporting member 2 which issandwiched between and which is bonded to the first and second filteringmembranes 3.

[0013] The first and second filtering membranes 3 can be made from amaterial selected from a group consisting of polytetrafluoroethylene(PTFE), glass fiber paper, and melt blown non-woven fabric. Preferably,the first and second filtering membranes 3 are polytetrafluoroethylenemembranes.

[0014] The air-permeable supporting member 2 is preferably a non-wovenfabric that is made from a material selected from a group consisting ofpolyethylene, polypropylene, polyamides, polyesters, aromaticpolyamides, and composite materials thereof. Preferably, the non-wovenfabric is made from a composite material of two fibrous materials havingdifferent melting points. Preferably, the two fibrous materials arerespectively polyethylene terephthalate and modified polyethyleneterephthalate.

[0015] The laminate of the air filter of FIG. 2 is prepared by hotpressing the first and second filtering membranes 3 and the supportingmember 2. In case the supporting member 2 is made from the compositematerial, the fibrous material having the lower melting point is fused,thereby causing the filtering membranes 3 to be bonded to the supportingmember 2.

[0016]FIGS. 3 and 4 respectively illustrate second and third preferredembodiments of this invention. The laminate of the air filter of thisinvention can include a plurality of filtering membranes 3 andsupporting members 2 alternately laminated to each other, as bestillustrated in FIGS. 3 and 4.

[0017] The following Example and Comparative Example illustrate theadvantages of the air filter of this invention as compared to those ofconventional air filters.

EXAMPLE 1

[0018] Two polytetrafluoroethylene membranes, each of which has acollecting efficiency of 99.69%, and a supporting member 2 made of thecomposite material of polyethylene terephthalate and modifiedpolyethylene terephthalate were laminated to form the air filter of FIG.2. The air filter was subjected to performance test to measure thecollecting efficiency and the pressure drop thereof by passingtherethrough an air stream that contains 0.26 micron NaCl particles. Theair stream was kept at a constant flow rate of 5.3 cm/s (32 liters/min).The resulting values of the collecting efficiency and the pressure dropare respectively 99.999% and 27 mmH₂O.

COMPARATIVE EXAMPLE 1

[0019] A conventional air filter of FIG. 1 was prepared in this Example.The polytetrafluoroethylene membrane employed in this conventional airfilter has a collecting efficiency higher than that of thepolytetrafluoroethylene membrane employed in the aforesaid Example 1.Conditions for measuring the collecting efficiency and the pressure dropof this conventional air filter were the same as those in Example 1. Theresulting values of the collecting efficiency and the pressure drop wererespectively 99.996% and 148 mmH₂O.

[0020] Comparison between the Example 1 and the comparative Example 1shows that with two PTFE membranes, each of which has a lower collectingefficiency than that of the PTFE membrane of the conventional airfilter, the overall collecting efficiency of the air filter in Example 1can reach up to 99.999% as compared to 99.996% for the conventional airfilter, while the overall pressure drop of the air filter can besignificantly reduced to 27 mmH₂O. It is herein proposed that bymaintaining the same overall collecting efficiency, the overall pressuredrop can be significantly reduced for the air filter that has a greaternumber of PTFE membranes, each of which has a lower collectingefficiency.

[0021] With the invention thus explained, it is apparent that variousmodifications and variations can be made without departing from thespirit of the present invention.

I claim:
 1. An air filter for collecting suspended fine particles in aclean room, said air filter comprising: at least a laminate includingporous first and second polytetrafluoroethylene membranes and anair-permeable supporting member that is sandwiched between and that isbonded to said first and second polytetrafluoroethylene membranes. 2.The air filter of claim 1, wherein said air-permeable supporting memberis a non-woven fabric.
 3. The air filter of claim 2, wherein saidnon-woven fabric is made from a material selected from a groupconsisting of polyethylene, polypropylene, polyamides, polyesters,aromatic polyamides, and composite materials thereof.
 4. The air filterof claim 2, wherein said non-woven fabric is made from a compositematerial of polyethylene terephthalate and modified polyethyleneterephthalate.